CLOSE TO THE DREDGE: PRECISE X-RAY C AND N ABUNDANCES IN λ ANDROMEDA AND ITS PRECOCIOUS RED GIANT BRANCH MIXING PROBLEM
CLOSE TO THE DREDGE: PRECISE X-RAY C AND N ABUNDANCES IN λ ANDROMEDA AND ITS PRECOCIOUS RED GIANT BRANCH MIXING PROBLEM
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接近挖泥船：λ仙女座的精确 X 射线 C 和 N 丰度及其早熟红巨星分支混合问题
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发表时间：
2011
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通讯作者：
R. Stancliffe
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作者：
J. Drake;B. Ball;J. Eldridge;J. Ness;R. Stancliffe
在 RS CVn 型双星 λ And 的主星日冕中形成的类 H C 线和 N 线的钱德拉 LETG+HRC-S 和 XMM-牛顿 RGS 光谱，是一个轻度贫金属的 G8 III-IV 首次上升巨星，于约 50 Myr 前完成了挖掘，已被用于对其表面 C/N 比进行精确测量。我们得到了正式结果 [C/N] = 0.03 ± 0.07，这是旧盘巨星的典型结果，与恒星 ≲ 1 M☉ 的标准挖掘理论一致。相比之下，这些恒星作为一个整体，包括 λ 和 ，其 12C/13C ≲ 20 远低于标准模型的预测。我们表明，旧盘巨星的丰度与包括从红巨星分支光度函数“凹凸”开始的温盐混合在内的模型一致。相反，λ 和 表明 12C/13C 异常可以在挖掘后立即出现，这与红巨星分支上额外混合的当前模型相反。在最近的 RS CVn 型双星的其他 C 和 N 丰度结果的背景下，这种异常似乎可能与强磁活动、快速旋转或两者兼而有之，而不是与接近的双星本身有关。
Chandra LETG+HRC-S and XMM-Newton RGS spectra of H-like C and N lines formed in the corona of the primary star of the RS CVn-type binary λ And, a mildly metal-poor G8 III–IV first ascent giant that completed dredge-up ∼50 Myr ago, have been used to make a precise measurement of its surface C/N ratio. We obtain the formal result [C/N] = 0.03 ± 0.07, which is typical of old disk giants and in agreement with standard dredge-up theory for stars ≲ 1 M☉. In contrast, these stars as a group, including λ And, have 12C/13C ≲ 20, which is much lower than standard model predictions. We show that the abundances of the old disk giants are consistent with models including thermohaline mixing that begins at the red giant branch luminosity function “bump.” Instead, λ And indicates that the 12C/13C anomaly can be present immediately following dredge-up, contrary to current models of extra mixing on the red giant branch. In the context of other recent C and N abundance results for RS CVn-type binaries it seems likely that the anomaly is associated with either strong magnetic activity, fast rotation, or both, rather than close binarity itself.